Conventionally, an obstacle detection system that smoothes out dispersion in the oscillation frequency of a microphone used for an ultrasonic sensor is proposed in, for example, US2003/0034883 A1 (JP 3521892 B2). In this obstacle detection system, first a microphone is assembled in a sensor circuit board, and then an obstacle for a test is disposed against the microphone. Subsequently, the obstacle detection system transmits an ultrasonic wave to this obstacle, finds an oscillation frequency of the ultrasonic wave that has a highest peak voltage of its reflected wave, and stores the oscillation frequency in a nonvolatile memory.
In detecting an obstacle actually, an ultrasonic wave of the oscillation frequency stored in this nonvolatile memory is transmitted, whereby the dispersion in the oscillation frequency of the microphone is smoothed out. Therefore, a stringent specification to the microphone becomes unnecessary.
Generally, the ultrasonic sensor is equipped with a filter circuit that filters signals of the reflected wave and eliminates frequency components that deviate from the resonance frequency of the microphone. The center frequency of this filter circuit can be set up in advance based on a resonance frequency that is determined simply by nominal specification without considering the dispersion in the specification of the microphone. For example, when the resonance frequency of the microphone is 40 kHz, the center frequency is set up in advance based on that value.
In contrast to this, in a case where specifications of the microphones are completely different, for example, there exist a plurality of specifications regarding the resonance frequency of the microphone, such as 40, 50 and 60 kHz, it is necessary to prepare filter circuits corresponding to respective specifications.
That is, when an ultrasonic sensor is installed on a vehicle and used to detect obstacles existing in the surroundings of the vehicle, it is desired to alter a detection range and a detection span of the ultrasonic sensor according to the type of a vehicle, installation positions, etc. Therefore, it is necessary to use microphones having different specifications from one another for respective purposes.
Thus, when microphones having different specifications are used, it is necessary to prepare special filter circuits each corresponding to the specification of each microphone. The hardware including the filter circuit therefore lacks general versatility.